Rafter Calculator
Calculation Results
Common Rafter Run: 0 inches
Common Rafter Length: 0 inches
Total Rafter Length: 0 inches
Roof Pitch: 0/12
Roof Pitch (degrees): 0 °
Plumb Cut Angle: 0 °
Level Cut Angle: 0 °
Unit Rise (per 12″ run): 0 inches
Unit Length (per 12″ run): 0 inches
Calculation Results
Error: Total Span is too small or ridge thickness is too large for a valid run. Please adjust inputs.'; return; } var commonRafterLength = Math.sqrt(Math.pow(commonRafterRun, 2) + Math.pow(totalRiseInches, 2)); var totalRafterLength = commonRafterLength + rafterOverhang; // Angle Calculations var roofPitchRadians = Math.atan(totalRiseInches / commonRafterRun); var roofPitchDegrees = roofPitchRadians * (180 / Math.PI); var plumbCutAngle = roofPitchDegrees; var levelCutAngle = 90 – roofPitchDegrees; // Unit Calculations (per 12 inches of run) var unitRise = (totalRiseInches / commonRafterRun) * 12; var unitLength = Math.sqrt(Math.pow(12, 2) + Math.pow(unitRise, 2)); var roofPitchRatio = unitRise.toFixed(2) + "/12"; // Display Results document.getElementById('commonRafterRun').innerText = commonRafterRun.toFixed(2); document.getElementById('commonRafterLength').innerText = commonRafterLength.toFixed(2); document.getElementById('totalRafterLength').innerText = totalRafterLength.toFixed(2); document.getElementById('roofPitchRatio').innerText = roofPitchRatio; document.getElementById('roofPitchDegrees').innerText = roofPitchDegrees.toFixed(2); document.getElementById('plumbCutAngle').innerText = plumbCutAngle.toFixed(2); document.getElementById('levelCutAngle').innerText = levelCutAngle.toFixed(2); document.getElementById('unitRise').innerText = unitRise.toFixed(2); document.getElementById('unitLength').innerText = unitLength.toFixed(2); } // Calculate on page load with default values window.onload = calculateRafter;Understanding Rafters and Roof Framing
Rafters are the backbone of any sloped roof, forming the structural framework that supports the roof decking, insulation, and roofing materials. Accurate rafter calculation is crucial for ensuring the structural integrity, weather resistance, and aesthetic appeal of a building. Whether you're building a shed, an addition, or a new home, understanding the geometry of rafters is fundamental to successful construction.
What is a Rafter?
A rafter is one of a series of sloping timbers that extend from the ridge or hip to the wall plate, providing the main support for the roof covering. Common rafters run perpendicular to the ridge board and wall plates, forming the primary slope of the roof. Other types include hip rafters, valley rafters, and jack rafters, each serving specific roles in more complex roof designs.
Why Accurate Rafter Calculation Matters
- Structural Integrity: Incorrectly sized or cut rafters can lead to a weak roof structure, susceptible to sagging, collapse under snow load, or damage from high winds.
- Weatherproofing: A precisely framed roof ensures proper drainage, preventing water pooling and leaks that can cause significant damage to the building's interior.
- Material Efficiency: Accurate calculations minimize waste, saving on material costs and reducing construction time.
- Aesthetics: A well-framed roof looks professional and contributes to the overall curb appeal of the structure.
- Code Compliance: Building codes often specify minimum requirements for rafter dimensions, spacing, and connections, which depend on accurate calculations.
Key Rafter Terminology Explained
Our Rafter Calculator uses several key terms to determine the precise dimensions and angles for your roof:
- Total Span: This is the total horizontal distance measured from the outside edge of one wall plate to the outside edge of the opposite wall plate. It defines the overall width of the roof structure.
- Total Rise: The total vertical distance from the top of the wall plate to the top of the ridge board. This measurement dictates the height of your roof.
- Rafter Overhang: The portion of the rafter that extends horizontally beyond the exterior wall plate. This creates the eaves of the roof, providing shade and protecting the walls from rain.
- Ridge Board Thickness: The thickness of the horizontal board at the peak of the roof where the tops of the rafters meet. This dimension is crucial for calculating the true run of the common rafter.
- Common Rafter Run: The horizontal distance from the outside edge of the wall plate to the center of the ridge board. For a symmetrical gable roof, this is half of the total span minus half the ridge board thickness.
- Common Rafter Length: The actual length of the rafter from the outside edge of the wall plate to the center of the ridge board, measured along its slope. This is the hypotenuse of a right triangle formed by the run and the rise.
- Total Rafter Length: The common rafter length plus the rafter overhang. This is the total length of lumber you'll need for each common rafter.
- Roof Pitch (X/12): Expressed as a ratio, this indicates how many inches the roof rises vertically for every 12 inches it runs horizontally. For example, a 6/12 pitch means the roof rises 6 inches for every 12 inches of horizontal run.
- Roof Pitch (degrees): The angle of the roof's slope measured in degrees from the horizontal. This is essential for setting your saw for accurate cuts.
- Plumb Cut Angle: The angle of the cut made at the top of the rafter where it meets the ridge board, and also at the end of the rafter for the fascia board. This cut is vertical when the rafter is in place.
- Level Cut Angle: The angle of the cut made at the bottom of the rafter for the birdsmouth seat cut, where the rafter rests on the wall plate. This cut is horizontal when the rafter is in place.
- Unit Rise (per 12″ run): The vertical rise of the roof for every 12 inches of horizontal run. This is the 'X' in the X/12 pitch ratio.
- Unit Length (per 12″ run): The diagonal length of the rafter for every 12 inches of horizontal run. This is useful for quickly estimating rafter length based on run.
How to Use the Rafter Calculator
Using this calculator is straightforward:
- Measure Your Span: Determine the total horizontal distance your roof will cover, from the outside of one wall plate to the outside of the other. Input this in feet and inches.
- Determine Your Rise: Decide on the desired vertical height of your roof from the wall plate to the ridge. Input this in feet and inches.
- Specify Overhang: Enter the desired length of your rafter overhang in inches.
- Input Ridge Thickness: Measure the thickness of the ridge board you plan to use in inches.
- Click "Calculate Rafters": The calculator will instantly provide all the necessary dimensions and angles for cutting your common rafters.
Example Calculation
Let's say you're building a small shed with the following specifications:
- Total Span: 10 feet 0 inches
- Total Rise: 3 feet 0 inches
- Rafter Overhang: 12 inches
- Ridge Board Thickness: 1.5 inches
Using the calculator with these inputs, you would get results similar to these:
- Common Rafter Run: 59.25 inches
- Common Rafter Length: 66.97 inches
- Total Rafter Length: 78.97 inches
- Roof Pitch: 7.26/12
- Roof Pitch (degrees): 31.20 °
- Plumb Cut Angle: 31.20 °
- Level Cut Angle: 58.80 °
- Unit Rise (per 12″ run): 7.26 inches
- Unit Length (per 12″ run): 13.99 inches
These precise measurements allow you to mark and cut your rafters with confidence, ensuring a strong and well-built roof.